Shop / engineering: pulley / block-and-tackle
Pulleyblock & tackle.
One load, three ways to hang it. Count the rope parts that hold the weight. That count is the ideal mechanical advantage.
LoadEffort Ideal advantage. It matches the rope parts holding the 10 lb load.
Setting the pulleys on the bench…
Lesson Shop / engineering: pulley / block-and-tackle
FixedChanges direction
MovableRides with the load
Load10 lb
Effort10.0 lb
Standing endTied off
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Exh 04Pulley10 lb load
Brass rope holds the load. The pale rope is the haul. Drag to look.
Count the ropes.
The load stays 10 lb. A fixed pulley turns the pull down. A movable pulley shares the weight. A block and tackle adds parts.
What a pulley is for
Weight or distance
A pulley is a wheel on an axle with a rope in the groove. The load is 10 lb. The effort is what you pull.
Ideal mechanical advantage equals the number of rope parts holding the load. More parts, less effort, more rope for the same lift.
The brass ropes are the supporting parts. The pale rope is the haul. This is an ideal model: no friction, so the effort is the load divided by the part count.
Start on Fixed. Effort matches the 10 lb load, and you pull down. Switch to Movable. Effort falls to 5 lb and the arrow points up. Open Block and tackle and set 4 parts. Effort is 2.50 lb. Watch one haul: the rope travels farther than the load.
Count theparts.
Use this page next to a single sheave and a short tackle. The classroom use is pulley and block-and-tackle, and the force you trade for rope.
03.1
Learning objectives
- Point to a fixed pulley and a movable pulley.
- Count the rope parts that hold the load.
- State that ideal mechanical advantage equals that count.
- Compute effort for a 10 lb load, and say what extra rope buys.
03.2
Classroom loop
- Cover the readout. On Fixed, students predict the effort for 10 lb.
- Uncover 10 lb. Switch to Movable. Predict again, then check 5 lb and the upward pull.
- Open Block and tackle at 4 parts. Count the brass ropes before reading 2.50 lb.
- Ask how many inches of rope lift the load one inch.
03.3
Shop examples
Fixed: a flag halyard, a clothesline, a window sash. Movable: a hay hoist, a bosun’s chair. Block and tackle: a barn beam, a sail halyard, an engine lift. A real rope adds friction, so the shop effort is a little higher than the ideal.